There is known a system in which a plurality of battery storage systems are connected with an energy management system (i.e., charge/discharge instruction apparatus) that aggregates such battery storage systems. Given such a system, consider a large-scale system having a three-layer structure, in which a plurality of energy management systems (i.e., higher-order charge/discharge instruction apparatuses) are added as an upper layer for the charge/discharge instruction apparatus. In this case, the charge/discharge instruction apparatus functions as a gateway and the higher-order charge/discharge instruction apparatuses use the battery storage systems via the gateway.
To perform charge/discharge control using the battery storage systems by the gateway or the higher-order charge/discharge instruction apparatuses, the gateway needs to collect information of the battery storage systems. As a collection method, there is a method that the gateway communicates with each of the battery storage systems for a data read request and its response at any time. In this method, there may occur a problem of large volume of communicates, resulting in an increase in network load between the gateway and each battery storage system and lowered communication speed. As the number of battery storage systems as a lower layer for the gateway increases, this problem becomes serious. As another method, in a method in which a battery storage system reports information to a gateway by push-type communication, the similar problem may occur depending on the report frequency or the number of report Items.